Another curious data point on plug gap
While attempting to balance the carbs on N73HR, we found it very difficult to get rid of some rather bad vibration. After several iterations, we finally decided to pull a plug to look for fouling or some such. What we found was that the plug was gapped at 0.018--quite a ways off from spec, and the 0.027 for just-out-of-the-box plugs. Pulling the remainder of the plugs, ALL were found to be pretty close to the 0.018 range. After re-gapping the works, and another tweak of the carbs, the engine vibration subsided considerably. A further minor tweak to balance both prop blades to within 0.2 degrees seemed to help a bit more, and we suspect that a prop balancing will further help smooth things out. None of the local Lycoming/Continental N.A.E.s (Noted Authority on Everything) could come up with a satisfying theory as to why the gapping affected engine vibration so much.
Idle speculation/faith-promoting rumors on the subject would be appreciated.
